1. Technical Field
This disclosure relates to an image processing apparatus, an image processing method, and a computer product.
2. Description of the Related Art
Color matching systems (CMSs) have been developed that perform color matching process on color image data stored in a computer to output it to a color output device such as a color printer.
Basically, in the CMS, image data represented by a red, green, and blue (RGB) signal is converted to a color signal used in an output device and metrically matching the image data. Colors appearing on display devices, however, cannot be reproduced as they are because a color reproduction range (color gamut) of an electrophotographic printer or an ink-jet printer is significantly smaller than that of display devices. Technology for mapping colors not reproducible by an output device to reproducible colors (hereinafter, “gamut compression”) has been known and proposed as various methods. For example, one such technology reproduces a color not reproducible by an output device, with a color having a minimum color difference calculated by changing the lightness, chroma, and hue weighting of a color reproducible by the output device. Another such technology causes, by setting a projection target on an achromatic axis or on a chromatic axis of a hue matching a hue of an input color signal, colors outside a gamut of an output device to have a uniform hue, thereby compressing and mapping the colors to fall within the gamut of the output device.
Because a color reproduction range of an electrophotographic printer or an ink-jet printer is significantly smaller than that of display devices, the gamut compression technology causes a large difference in reproduced colors even with a slight difference in setting conditions of the gamut compression methods. Further, due to inconsistency of color reproduction ranges of an output device, reproduced colors may appear different even by applying the same gamut compression method.
For this reason, color conversion methods have been proposed for matching colors of a plurality of output devices.
For example, Japanese Patent Application Laid-open No. 2000-50092 discloses color conversion including converting an input color signal to a standard input color space (sRGB), converting the standard input color space to a standard output color space (sCMYK), and converting the standard output color space to an output color space of a printer. Japanese Patent No. 3513334 discloses a conventional technology in which colors are processed commonly for a plurality of output devices in a device-independent color space, and then converted to device-specific color signals by using device-specific profiles. In this publication, colors are processed commonly for a plurality of devices by finding a common color reproduction range reproducible by any of the output devices, and then converting the colors by using color profiles for processing the gamut of the colors to the common color reproduction range. In this way, colors of the devices are equalized.
Japanese Patent Application Laid-open No. 2005-269200 discloses another conventional technology in which a color space is compressed in a color reproduction range of an output device of a type other than that of a target output device, and then compressed in a color reproduction range of the target output device. Japanese Patent Application Laid-open No. 2002-252785 discloses still another conventional technology in which a gamut is compressed based on a virtual color reproduction range that covers color reproduction ranges of a plurality of output devices, and then compressed in a color reproduction range of each of the devices, allowing colors to be perceived as almost the same colors. Japanese Patent Application Laid-open No. 2004-153340 discloses gamut compression including reading respective target colors of a plurality of output devices, and then defining a common hue line. Japanese Patent Application Laid-open No. 2005-295047 discloses still another conventional technology in which a virtual color reproduction range is determined that has a proper size covering color reproduction ranges of a plurality of output devices, and that satisfies certain criteria with regard to a content rate of a reference color.
With the conventional technology disclosed in Japanese Patent No. 3513334, color reproduction ranges of the output devices are analyzed, and colors are reproduced by using a common color reproduction range reproducible by all the output devices. Because the color reproduction range is adjusted to a smaller color reproduction range of the devices, color reproduction capabilities of the devices cannot be fully utilized (FIG. 33A).
In the conventional technology disclosed in Japanese Patent Application Laid-open No. 2005-269200, an output device having a preferable color reproduction range is used as a reference output device. This method also fails to fully utilize a color reproduction capability of an output device having a larger color reproduction range than that of the reference output device.
With the conventional technologies disclosed in Japanese Patent Applications Laid-open Nos. 2002-252785, 2004-153340, and 2005-295047, colors are subjected to common gamut processing by using a virtual color reproduction range that covers color reproduction ranges of a plurality of output devices, and then converted to colors within the color reproduction ranges of the devices. This method causes a difference in size and shape between the virtual color reproduction range and the color reproduction ranges of the output devices. As a result, reproduced colors vary depending on a color conversion system converting colors from the common reproduction range to reproduction ranges of the output devices, failing to equalize the colors (FIG. 33B).
As such, the conventional technologies do not provide desirable color reproduction when common color reproduction information differs from color reproduction ranges of output devices with regard to shape and size. Further, a virtual color reproduction range for defining a target reproduction color significantly differs from the color reproduction ranges of output devices, causing a large difference between the target reproduction color and optimum output colors for the output devices.